1. Field of the Invention
This invention generally relates to a flame sensor. More particularly, this invention relates to a flame sensor capable of detecting a flame in places where solar rays and artificial rays of light are present without being affected by such rays of light.
2. Description of the Related Art
To detect a flame, there is a convenient method that detects resonance radiation generated by a high-temperature carbonic acid gas contained in the flame, as is well known in the art. A line spectrum of resonance radiation of the carbonic acid gas includes many wavelengths. To discriminate the line spectrum from ordinary artificial illumination and solar rays, it is appropriate to utilize a spectral line within the range of the infrared region or the ultraviolet region for detecting the flame.
Optical components belonging to both these regions do not much exist in artificial rays of light such as illumination, so disturbance by external light when sensing a flame is less in these regions.
To detect a flame in the presence of solar rays, a conventional method detects the line spectrum due to resonance radiation of the carbonic acid gas generated by the flame. To discriminate a continuous spectrum, such as solar rays and artificial light, from the line spectrum of the flame, this method compares and computes a plurality of outputs obtained from a monochromatic filter having a narrow-band that permits the passage of only the line spectrum of the flame and from monochromatic filters of a plurality of narrow-bands, which permit the passage of rays of light having one or a plurality of wavelengths, and the method discriminates whether light is the line spectrum of the flame or the continuous spectrum of the solar rays.
Another method utilizes flicker of light generated by the flame and detects the occurrence of the flame.
Among conventional methods that utilize resonance radiation of the carbonic acid gas, the method using the filter requires at least three monochromatic filters to achieve a flame sensor providing a small number of erroneous detections and capable of reliably sensing a flame. In addition, a computation circuit for sensing is complicated, and the flame sensor is unavoidably expensive.
Flame sensors using two or less filters involve the problem that the number of erroneous detections is great. Though economical, flame sensors utilizing the flicker of the flame also involve the problem that the number of erroneous detections is great. Therefore, the applicant of the present application has already proposed a flame sensor capable of reliably detecting a flame with equivalent certainty to the conventional flame sensors using three filters, and a flame sensor using three filters but using a simple computation circuit.
Solar rays, artificial rays or radiation from a stove emit not only visible rays, but also radiation in the infrared regions. However, this radiation is a continuous spectrum. In contrast, the spectrum of resonance radiation of the carbonic acid gas generated by the flame is a line spectrum in which energy concentrates in extremely narrow regions. Therefore, the technology described above utilizes the difference between the continuous spectrum and line spectrum for detecting the flame.
This technology, shown in FIG. 11, uses a broadband filter for permitting the passage of light of a band (W10) broader than a spectral line(W20) of resonance radiation of the carbonic acid gas generated by the flame and a narrow-band filter for permitting the passage of only the spectral line of resonance radiation of the carbonic acid gas, and has the band center of the broadband filter in alignment with that of the narrow-band filter. Intensity (optical energy) of light from the flame passing through these two filters is divided by the bandwidth of each filter to determine mean intensities.
When the intensity of the spectrum of light passing through the filters is a straight line-like continuous spectrum, energy of the rays of light passing through the two filters is proportional to the transmission bandwidth. Therefore, the mean intensities obtained by dividing this energy by the bandwidth are equal for the two filters.
However, when the rays of light passing through the filters are the line spectrum of resonance radiation of the carbonic acid gas, both of these two filters allow this line spectrum to pass therethrough and transmission energy is substantially equal. However, optical energy of the light passing through the broadband filter is divided by a greater bandwidth to calculate the mean intensity, whereas optical energy of the light passing through the narrow-band filter is divided by a smaller bandwidth. Consequently, a difference develops between these two mean intensities.
Therefore, the flame can be detected by judging whether or not a difference between the two mean intensities exceeds a threshold value.
In the technology described above, however, the band center of the broadband filter and the band center of the narrowband filter are in alignment with each other. Therefore, when the straight line-like continuous spectrum passes through the filters, the difference of the mean intensities is 0. To discriminate the straight line-like continuous spectrum from other spectra, the threshold value must be set to a small value near 0. However, it is difficult, from the aspect of production, to have the band center of the broadband filter in alignment with the band center of the narrowband filter. If the band centers of these two filters are not coincident, the difference of the mean intensities will not become 0 even when the straight line-like spectrum passes, resulting in inviting the occurrence of erroneous detections.
The explanation given above holds also true of the case where a first filter for allowing the passage of only light of the spectral line of the resonance radiation of the carbonic acid gas generated by the flame and a second filter for allowing the passage of light of a broader band than the spectral line are employed, the second filter being disposed in such a way that its band center is coincident with that of the spectral line, and the quantities of energy passing through these two filters is subtracted to detect a flame.
To solve the problems described above, the present invention aims to provide a flame sensor that can be easily produced and can accurately detect a flame.
A first aspect for accomplishing the objects described above provides a flame sensor that comprises a narrowband filter which passes only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a broadband filter which passes light of a band broader than the band corresponding to the line spectrum, and which has a band center different from a band center of the band corresponding to the line spectrum; a first light reception device which converts light passing through the narrowband filter to an electric signal; and a second light reception device which converts light passing through the broadband filter to an electric signal.
When the spectrum of the light passing through the filter is the continuous spectrum, energy of the rays of light passing through the two filters, the broadband filter and the narrow-band filter, is substantially proportional to the transmission bandwidth. Therefore, a difference between the mean intensities obtained by dividing this energy by each bandwidth is less than a predetermined value. The source of the difference between the mean intensities include the shape of the intensity distribution of the spectrum of rays of light passing through the filters and the distance between the band centers of the two filters.
In contrast, when only rays of light of a flame are present, the spectrum passing through the broadband filter and the narrow-band filter is mainly only the spectral line because the spectrum of the flame is the line spectrum, and energies passing through the broadband filter and the narrow-band filter are substantially equal to each other. Therefore, a mean intensity obtained by dividing energy of the spectrum passing through the broadband filter by the transmission bandwidth thereof is smaller than a mean intensity obtained by dividing energy of the spectrum passing through the narrow-band filter by the transmission bandwidth.
Therefore, a flame can be detected by judging whether a difference between the mean intensities of the electric signals in the transmission band of the narrow-band filter and in the transmission band of the broadband filter, that is, the difference obtained by subtracting the mean intensity of the rays of light passing through the broadband filter from the mean intensity of the rays of light passing through the narrow-band filter, exceeds a predetermined value. Detection of the flame can be achieved by providing a judging device for judging whether or not the difference between the mean intensities of the electric signals exceeds a predetermined value. A digital circuit including a differential amplifier or a CPU can compute this difference between the mean intensities.
A second aspect of the invention provides a flame sensor that comprises a first filter having a predetermined band for passing light, and having, within the predetermined band, a band blocking light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a second filter having a band substantially the same as the predetermined band, passing light of a band inclusive of the band corresponding to the line spectrum, and having a band center different from a band center of the band corresponding to the line spectrum; a first light reception device which converts light passing through the first filter to an electric signal; and a second light reception device which converts light passing through the second filter to an electric signal.
When a spectrum of light passing through the filters is a continuous spectrum, energy of the light passing through the two filters is substantially proportional to the transmission bandwidth. When the spectrum is a line spectrum, energy passing through the two filters is substantially equal. Therefore, a flame can be detected by judging whether or not a difference between mean intensity of a signal obtained by subtracting an electric signal converted by the first light reception device from an electric signal converted by the second light reception device, that is, a difference obtained by subtracting the mean intensity of the electric signal converted by the first light reception device from the mean intensity, exceeds a predetermined value. This flame detection can be achieved by providing judgment device for judging whether or not the difference between the mean intensity of the signal as obtained by subtracting the electric signal converted by the first light reception device from the electric signal converted by the second light reception device in the line spectrum band and the mean intensity of the electric signal converted by the second light reception device, the mean intensity for the transmission band of the second filter, exceeds a predetermined value.
Lead selenide or a thermopile or pyroelectric-type light reception device can be used for the light reception devices of the first and second aspects. The existence/absence of the flame may be judged from the intensity of the line spectrum of resonance radiation of the carbonic acid gas obtained on the basis of the two electric signals obtained from the two filters, or may be judged from an AC component, caused by flicker of light of the flame, in the signal of the line spectrum of resonance radiation of the carbonic acid gas obtained by these two filters. Furthermore, flame detection can be done effectively when a dome-shaped diffusive transparent plate is used as a light reception window of the flame sensor.
In the first and second aspects described above, the predetermined value is preferably varied in accordance with the intensity of the electric signals outputted from the second light reception device. It is further preferred to increase the predetermined value with an increase of intensity of the electric signal outputted from the second light reception device.
A third aspect of the present invention is a flame sensor comprising: a narrowband filter which passes only light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a broadband filter which passes light of a band broader than the band corresponding to the line spectrum; a first light reception device which converts light passing through the narrowband filter to an electric signal; a second light reception device which converts light passing through the broadband filter to an electric signal; and a preventing member for preventing generating a secondary radiation at the narrowband filter and the broadband filter, the preventing member being provided at a front side of the narrowband filter and the broadband filter.
A fourth aspect of the present invention is a flame sensor comprising: a first filter having a predetermined band for passing light, and having, within the predetermined band, a band blocking light of a band corresponding to a line spectrum of carbonic acid gas resonance radiation generated by a flame; a second filter having a band substantially the same as the predetermined band, passing light of a band inclusive of the band corresponding to the line spectrum, and having a band center different from a band center of the band corresponding to the line spectrum; a first light reception device which converts light passing through the first filter to an electric signal; a second light reception device which converts light passing through the second filter to an electric signal; and a preventing member for preventing generating a secondary radiation at the first filter and the second filter, the preventing member being provided at a front side of the first filter and the second filter.
In the third aspect of the present invention, the preventing member for preventing generating the secondary radiation at the narrowband filter and the broadband filter is provided at the front side of the narrowband filter and the broadband filter. In the fourth aspect of the present invention, the preventing member for preventing generating the secondary radiation at the first filter and the second filter is provided at the front side of the first filter and the second filter. Accordingly, in the light entering into the frame sensor, the light incidents to each filter after the light passes through the preventing member. Therefore, the second radiation due to sunlight entering the filter can be prevented.
In the third and fourth aspects of the present invention, the preventing member is preferably a silicon plate.
Moreover, in the third aspect, the circuit for calculating a mean intensity of the first light reception device, obtained such that a light energy passing through the narrowband filter is divided by bandwidth of the narrowband filter, and a mean intensity of the second light reception device obtained such that a light energy passing through the broadband filter is divided by bandwidth of the broadband filter, is preferably further provided. Also, in the fourth aspect, a circuit for calculating a mean intensity of the first light reception device, obtained such that a light energy passing through the first filter is divided by bandwidth of the first filter, and a mean intensity of the second light reception device obtained such that a light energy passing through the second filter is divided by bandwidth of the second filter, is preferably further provided.